Pathogenic agents such as viruses, bacteria, metazoan parasites and human cancers have evolved elaborate strategies to defeat the host immune response. Such strategies often hamper efforts to develop successful vaccines against many pathogenic organisms.
Certain parasites have evolved an intracellular habitat that helps the parasites avoid the effects of antibody. Other parasites like the African trypanosomes use a process called antigenic variation to change the character of their surface coats. Still other pathogens have developed ways to suppress the host's immune response by releasing lymphocytotoxic factors.
According to yet another strategy the pathogen displays an immunodominant epitope that undergoes structural variation or antigenic drift. Early neutralizing antibody and/or cytotoxic T lymphocyte (CTL) responses which are raised against these epitopes represent an attempt by the host's immune system to reduce the titer of the dominant pathogenic phenotype. However, there is a lag-period between the time of infection and the appearance and effect of these immune responses. Moreover, antigenic drift of the immunodominant epitope results in these early neutralizing antibodies or CTL responses becoming ineffective against the pathogen.
The human immunodeficiency virus-1 (HIV-1) has evolved an exquisite strategy that it uses to evade, and so to destroy, the human immune system. None of the vaccine approaches that have been attempted to date have proved successful. One approach at vaccine production has centered on gp120/160 of HIV-1. Neutralizing antibodies can be raised against the dominant V3 domain of gp120/160. However, these neutralizing antibodies are not effective in preventing the continued growth of HIV-1 in vivo. Haigwood et al., AIDS Research and Human Retroviruses 6:855-69 (1990), produced a gp120/160 immunogen that did not harbor the dominant V3 domain by deleting the amino acids that comprised the V3 domain. This engineered protein was produced in a non-glycosylated form in yeast, denatured and used to immunize test animals. This approach failed to elicit a more conserved neutralizing response.
The influenza virus hemagglutinin antigen (HA) provides another example of a pathogen-encoded immunodominant antigen that is subject to antigenic drift. Indeed, variation in the antigenic structure of HA correlates with the periodic epidemics of respiratory disease that are caused by this virus. Under experimental conditions, the selective pressures imposed by propagating the virus in the presence of neutralizing antibodies have lead to the emergence of resistant variants. In one example, a mutation at position 63 of HA1 (D to N) resulted in the creation of a three amino acid motif that fit the consensus N-X-S/T. This motif serves as the signal for N-linked oligosaccharide addition to proteins that transit through the endoplasmic reticulum and golgi. The presence of a supernumerary carbohydrate blocked the interaction between the HA protein and the neutralizing antibody. This was confirmed by the finding that propagation of this mutant in the presence of tunicamycin, an inhibitor of N-linked glycosylation, restored antibody binding. Hence, a post-translational modification of a virally encoded epitope can interfere with the binding of neutralizing antibodies.
Gething et al., U.S. Pat. No. 5,041,376, discloses a method for shielding epitopes of proteins by incorporating N-linked oligosaccharide side chains using oligonucleotide mutagenesis. The contemplated use of the N-linked modifications of the proteins is to increase the circulation time of the antigens by decreasing their immunogenicity.